This invention relates to adaptive bias currents for magnetoresitive (MR) read heads and particularly to selecting a bias current for an MR head based on read error rates.
Magnetoresistive (MR) heads are employed in magnetic disc drives to read data from the storage disc. More particularly, the MR head employs an MR element whose resistance changes with changes in the confronting magnetic field. As the disc rotates adjacent the read head, changing magnetic fields due to recorded data on the disc moving past the MR element induces changes in the resistance of the MR element. A fixed bias current is applied to the head to generate a voltage across the head representative of the data. The voltage changes with the resistance changes to provide signals representing the data.
There is a continuing need to increase the recording capacity, and hence the density of data recording, on discs. Consistent with this need, efforts have been directed to more narrow data tracks and smaller track spacing. As track widths become more narrow and track spacing becomes smaller, the read heads also become more narrow, so as not to extend over the space between the tracks that might cause errors due to simultaneous reading of two or more data tracks.
As the dimensions of the MR heads become smaller, it becomes more difficult to control the dimensional variations to a reasonable tolerance. Consequently, smaller MR heads are subject to large relative dimensional variations, resulting in wide variations of the resistance of the heads. As a result, for a given bias current, different heads of the same disc drive might exhibit large differences in resistance ranges, resulting in a wide range of power dissipation between the heads. Thus, the bias current must be increased for heads whose MR elements have low resistance in order to achieve a design output voltage. Conversely, if the MR element has a high resistance, the bias current must be reduced so that the head does not overheat due to increased power dissipation. Overheating the head leads to head failure.
It has been the practice to match MR heads for a given disc drive so that the same bias current level is applied to all of the heads in the drive. Thus, the head characteristics are measured and the heads are matched with other like heads for the disc drive. However, while head matching solves the problem of bias current levels at the time of manufacture, different head react to environmental conditions differently, so that a condition, such as temperature, that might alter the resistive characteristics of the heads may affect different heads of the same disc drive differently, resulting in head failure.
The present invention is directed to an adaptive bias current system whereby the bias current for a given MR head is adaptively selected for optimal performance of the head. Where plural recording surfaces, and hence plural read heads, are employed within the disc drive, the bias current for each MR head is optimally tailored to the head. Hence, the present invention is directed to adaptively tuning the bias current to the MR heads to address manufacturing variances and fluctuations in magnetoresistive properties of the head due to environmental conditions, such as temperature fluctuations.
The present invention addresses these and other problems, and offers other advantages over the prior art.
Broadly, the present invention provides a method of selecting a bias current for a magnetoresistive read head for a disc drive. A maximum and a minimum bias current value for the head are identified, from which a range of bias current values is derived. A plurality of read operations are iteratively performed using a different bias current value for each iteration over the range. A read error rate is measured for the result of each read iteration. The bias current is selected based on the read error rates.
In a preferred form of the invention, the bias current value is incrementally stepped over the range, and the bias current is selected based on the lowest read error rate. Where a single bias current value is associated with the lowest read error rate, the selected bias current has that bias current value. Where a plurality of bias current values are associated with the lowest read error rate, the selected bias current has a value between the highest and lowest bias current values associated with the lowest read error rate.
In one form of the invention, where a plurality of bias current values are associated with the lowest read error rate, the number of incremental bias current values in the range defined by the highest and lowest bias current values associated with the lowest read error rate is identified. If the number is odd, the bias current whose value equals the bias current value centered between highest and lowest bias current values of the range is selected. If the number is even, the bias current whose value equals the bias current value centered between (a) either the highest or lowest bias current value and (b) a bias current value incrementally adjacent the other of the highest and lowest bias current values.
In one embodiment of the invention, the maximum bias current value is identified by identifying a predetermined maximum power for the head. The bias current to the head is then varied to identify a bias current that generates a voltage across the head that exceeds a predetermined threshold voltage. The maximum bias current value is then calculated based on the predetermined maximum power, the identified bias current and the predetermined threshold voltage.
In another embodiment of the invention, the maximum bias current value is identified by identifying a predetermined maximum power for the head. A voltage across the head to a read amplifier is varied from high to low until the voltage triggers operation of the amplifier. The bias current at which the voltage across the head triggers operation of the amplifier is identified, and the maximum bias current is calculated based on the predetermined maximum power, the identified bias current and the voltage triggering operation of the amplifier.
In a preferred form of the invention, the minimum bias current value is identified by varying a bias current to the head from high to low, monitoring an ability of a read amplifier receiving a voltage across the head generated by the bias current to sustain operation of a synchronization clock, and identifying the value of the bias current when the read amplifier becomes unable to sustain operation of the clock.